The present invention relates to a piston for an internal combustion engine, and more particularly, to a piston for a cylinder injection (direct injection) type internal combustion engine such as a diesel engine or a gasoline engine that directly injects fuel into cylinders.
A conventional example of a piston will now be described. FIG. 11 is a cross-sectional view showing a combustion chamber of a diesel engine 100. FIG. 12 is a plan view of a piston body 102. As shown in FIG. 11, the diesel engine 100 includes a piston 101 that includes the piston body 102. The piston body 102 includes a head 104, two side walls 109, and two skirts 112. The crown of the head 104 includes a recess 105. Each side wall 109 includes a pin boss 110. The two skirts 112 are respectively located at a thrust side (Th side) and an anti-thrust side (ATh side) with respect to the axis of a piston pin 107. The thrust side (Th side) is the side of the piston 101 forced against the wall of the cylinder immediately after the piston 101 reaches the top dead center. In FIG. 11, the left side of the piston 101 is the thrust side (Th side), and the right side of the piston 101 is the anti-thrust side. The pin bosses 110 support the piston pin 107. An injector 114 is located above the piston 101 to inject fuel toward the recess 105. Japanese Laid-Open Utility Model Publication No. 6-4348 describes an example of such a piston including a piston body with a recessed crown.
The piston body 102 includes a lip 116 defined by the rim of the recess 105 in the head 104. In the power stroke of the diesel engine, the piston 101 receives combustion gas having a high temperature and a high pressure. Thus, referring to FIG. 12, a large tensile stress that acts in the sideward direction of the engine (directions indicated by arrows y1 in FIG. 12) is applied to the lip 116 at the end located toward the engine front (Fr direction) and the end located toward the engine rear (Rr direction). A large pressure stress that acts in the front and rear directions of the engine (directions indicated by arrows y2 in FIG. 12) is applied to the lip 116 at the end located toward the thrust direction (Th direction) and the end located toward the anti-thrust direction (ATh direction). Further, the peripheral portion of the head 104 including the lip 116 is where the temperature of the piston 101 becomes the highest. Thus, the material strength is apt to decrease at this portion. This may form cracks 118 in the crown of the piston body 102 that extend from the Fr direction end and Rr direction end of the lip 116.
In Japanese Laid-Open Utility Model Publication No. 6-4348, a plate formed from copper alloy is coupled to an inner top surface of the piston body. The lower surface of the plate includes fins. The plate, which includes the fins, cools the piston body to prevent the formation of cracks in the crown of the piston body. However, the plate is merely coupled to the inner top surface of the piston body. Thus, the plate does not effectively increase the flexural rigidity of the head in the thrust direction and the anti-thrust direction. Further, the plate does not effectively prevent the formation of cracks in the crown of the piston body.